Pulping is a complex process and lignin is very heterogenous material therefore to perform a more detailed study on lignin diffusion, we included an additional solvent fractionation step.

In this study, diffusion cell methodology is implemented to systematically investigate the transport of fractionated kraft lignin molecules through model cellulose membranes. A comprehensive and detailed study of lignin mass transport through cellulosic materials is, therefore, of great importance. However, our current understanding of the diffusion of lignin through cellulose and the influential parameter on this process is very limited. Mass transport of liberated lignin fragments from pits and fiber walls into black liquor is considered a determining step in the delignification process.